According to the Results of National Nutrition Survey published by Ministry of Health and Welfare, the common salt intake by a Japanese per day on and after Showa 50 (1975) was 11.5 g or more and particularly, the amount was 12.8 g in Heisei 5 (1993). On the other hand, since there is a correlation between the common salt intake per day and the incidence rate of hypertension, Ministry of Health and Welfare has recommended to control the common salt intake per day to be 10 g or less in order to prevent the incidence of hypertension and further the incidence of cerebral strokes. Also, in the United States, the common salt intake per day is restricted as in Japan and a draft of advice of US Joint Committee has proposed controlling the common salt intake ingested by a hypertension patient per day to be 6 g or less.
Moreover, it is said that there is also a correlation between the common salt intake and the mortality owing to stomach cancer. There is obtained data that the mortality owing to stomach cancer is high in the areas where the common salt intake is large, such as Toyama city and Hirosaki city, while the mortality owing to stomach cancer is low in the areas where the common salt intake is small, such as Beppu city and Okinawa city.
Although it is reported that dietary fiber such as alginate salt has a certain degree of sodium ion adsorption ability (Non-Patent Document 1), the adsorption ability is not yet sufficiently satisfactory.
Since excessive existence of common salt in the body as above adversely affects the human body, it has been desired to develop a new technology which effectively inhibits the absorption of common salt into the body and excretes excessively existing common salt outside the body.
As such a technology, metal salts of cellulose derivatives with metals other than sodium have been proposed (Patent Document 1).
On the other hand, as a technology for altering the properties of cellulose, substituent introduction such as esterification of cellulose and crosslinking of cellulose have been performed.
For example, in aforementioned Patent Document 1, a technology of introducing a functional group having cation-exchange ability as a substituent is proposed.
With regard to the crosslinking of cellulose, depending on the types of the crosslinking agent, methods for introducing the same (reaction conditions) and chemical structures thereof are diversified. However, generally, most of the methods utilize as a functional group the hydroxyl group at 6-position of the glucose skeleton which is a monomer unit. The most commonly known one is a compound having a glycerol skeleton or a 1,2-diol skeleton in the crosslinking structure.
Moreover, as a technology for modifying the water-holding property of cellulose, a technology of introducing a sulfate group and subsequently crosslinking the obtained cellulose sulfate is proposed (Non-Patent Document 2, Patent Document 2).
It is considered that the improvement of performance through the modification of cellulose depends on the crosslinking and the amount of the functional group introduced. However, since both of the crosslinking of cellulose and the introduction of the functional group are conducted to the three hydroxyl groups contained in the glucose unit of celluloses, it is difficult to increase the degree of substitution by the functional group in crosslinked cellulose derivatives having the functional group. Actually, it is very difficult to increase the degree of substitution to be a value exceeding 1.1. Particularly, in the case of introducing the functional group into a crosslinked cellulose, it becomes more difficult to increase the degree of substitution.
In this connection, as a technology for introducing a functional group into a crosslinked cellulose, a technology of reacting the cellulose with HClO3S in pyridine is reported (Non-Patent Document 3).    Patent Document 1: WO 01/051063    Patent Document 2: JP-T-2003-520302    Non-Patent Document 1: Journal of Home Economics of Japan, 1988, Vol. 39, No. 3, p. 187-195    Non-Patent Document 2: Ken-ichiro Arai, Hideki Gota, “Crosslinked Sodium Cellulose Sulfate as Highly Water-Absorbable Material”, SEN-I GAKKAISHI, 1993, Vol. 49, No. 9, p. 482-485    Non-Patent Document 3: J. PASTYR, L. KUNIAK, “PREPARATION AND PROPERTIES OF A CELLULOSE SULFATE CATION-EXCHANGER BASED ON POWDERED CROSS-LINKED CELLULOSE”, CELLULOSE CHEMISTRY AND TECHNOLOGY, 1972, 6, P. 249-254